Automobiles with diesel engines are often equipped with a selective catalytic reduction (SCR) exhaust treatment system used to reduce the amount of nitrogen oxide (NOx) in the engine's exhaust. Generally, in an SCR system, a reducing agent such as urea or diesel exhaust fluid (DEF) is injected into the engine's exhaust stream to cause a chemical reaction that converts NOx into nitrogen and water. Sensors, such as urea quality sensors, are sometimes installed in SCR system tanks to sense properties of the reducing agent held in the tanks. One challenge encountered when using urea or DEF as the reducing agent is that it freezes at around −11° C., and therefore it can be difficult to sense its properties in cold climates when stored in the tanks.